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Ch. 4 - Acids and Bases: Electron Flow
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 4 - Acids and Bases: Electron FlowProblem 79c
Chapter 3, Problem 79c

In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH2.
(c) Carbenes are Lewis acids. Based on your answers to (a) and (b), explain this observation.

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1
Carbenes are neutral species with the general formula R₂C: (or CH₂ in the simplest case). They have a divalent carbon atom with only six valence electrons, making them electron-deficient.
To understand why carbenes act as Lewis acids, recall the definition of a Lewis acid: a species that can accept a pair of electrons.
In the case of carbenes, the carbon atom has an empty p orbital due to its electron deficiency. This empty orbital can accept a pair of electrons from a Lewis base, fulfilling the criteria for a Lewis acid.
The electron-deficient nature of carbenes arises from their electronic configuration, where the carbon atom has two nonbonding electrons (a lone pair) and two covalent bonds, leaving the p orbital vacant.
Thus, the ability of carbenes to accept electron pairs into their empty p orbital explains why they are classified as Lewis acids.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Carbenes

Carbenes are neutral reactive intermediates that contain a carbon atom with only six valence electrons, making them electron-deficient. The simplest carbene, methylene (CH₂), has two unshared electrons and can act as a nucleophile or electrophile in chemical reactions. Their unique structure allows them to participate in various organic reactions, often leading to the formation of new bonds.
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Reaction with a simple carbene.

Lewis Acids and Bases

Lewis acids are defined as species that can accept an electron pair, while Lewis bases donate an electron pair. Carbenes, due to their electron-deficient nature, can accept electron pairs from Lewis bases, thus behaving as Lewis acids. This property is crucial in understanding their reactivity and interactions with other chemical species.
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The Lewis definition of acids and bases.

Reactive Intermediates

Reactive intermediates are transient species that form during the course of a chemical reaction and are typically unstable. They include carbenes, radicals, anions, and cations, each playing a significant role in reaction mechanisms. Understanding these intermediates is essential for predicting the outcomes of chemical reactions and the pathways they may follow.
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Related Practice
Textbook Question

In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH2.

(d) Carbenes are also Lewis bases. Based on your answers to (a) and (b), explain this observation.

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Textbook Question

Imidazole has two potentially basic sites (a and b). Of the two, where would you expect a proton to add preferentially?

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Consider the following drugs used to treat the indicated diseases. Would you expect the activity of these drugs to be impacted by a patient taking other medicines for acid reflux disease?

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Textbook Question

BCl3 has an empty p orbital, so it is a strong Lewis acid. Would you expect an amide to react with BCl3 at nitrogen or oxygen?

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